The invention relates to an antenna exciter for at least two different frequency bands, consisting of two tubular polarization switches to each of which two wave guides of rectangular cross section for the guidance of linearly polarized electromagnetic waves are connected, and of a feed horn, both polarization switches having a circular inside cross section, the diameter of the two polarization switches being different, the two polarization switches being arranged without axially intervening space one behind the other, the feed horn being arranged at the free end of the polarization switch having the larger inside diameter, the polarization switch with the smaller inside diameter having a constant inside diameter over its entire length, and the two wave guides being so connected at each polarization switch that the waves fed into same have their plane polarization perpendicular to each other, the two wave guides of the polarization switch having the larger inner diameter being so connected impinging radially on same at two axially spaced places which are 90.degree. apart in circumferential direction that the large axes of their cross section extend in the direction of the axis of the polarization switch (GB-OS No. 2,117,980).
Such antenna exciters are used, for instance, for the illuminating of directional antennas with parabolic reflector for directional communication, satellite communication or radio position finding. They can be used in this connection for the direct illuminating of the reflector or also for the illuminating thereof via a subreflector (Cassegrain principle). In this connection, "illumination" is intended to cover both directions of transmission of the electromagnetic waves and therefore both waves to be radiated and waves to be received.
Polarization switches for the illuminating of reflectors are known, for instance, from U.S. Pat. No. 3,864,688. They serve so to uncouple two linearly polarized electromagnetic waves which have been guided over connected wave guides that they do not interfere with each other. In these known polarization switches there is used for this purpose a circular-cylindrical length of tube into which two wave guides debouch alongside of each other, spaced apart in axial direction. The uncoupling of the two waves is effected by a plurality of pins which are shifted relative to each other or a twisted sheet-metal strip which is arranged in the length of pipe between the points of connection of the two wave guides. In this way a rotation of the one wave by 90.degree. is produced so that the two waves are perpendicular to each other. The need for disturbance-free guidance of two linearly polarized waves of the same frequency band can be satisfied therefore at some expense with this known arrangement.
An antenna exciter for two different frequency bands can be noted from U.S. Pat. No. 4,410,866. It has two polarization switches, each of which is designed for a different frequency band. This antenna exciter can, for instance, guide two waves of a frequency band of 3.7 to 4.2 GHz which are perpendicular to each other--in the following called the "4-GHz band"--and two waves of a frequency band of 5.925 to 6.425 GHz which are perpendicular to each other--in the following called the "6-GHZ band." In the tubular polarization switch for the 4-GHz band which is used in this connection, filters are installed which are intended to act as short circuit for the 4-GHz band so as to prevent a propagation of the waves in the wrong direction. The waves of the 6-GHz band, on the other hand, are not to be disturbed by the filters. The installation of these filters which consist of beryllium oxide and the filters themselves represent a considerable expense. They furthermore require precision manufacture. Between the two polarization switches there is furthermore arranged a conically extending transition piece by which the antenna exciter is made longer and heavier. Installation in an antenna system is thereby made more difficult. Furthermore, this transition piece also requires precision manufacture if no disturbing reflections are to be produced.
In the case of the antenna exciter of the aforementioned GB-OS No. 2,117,980 the two polarization switches are arranged directly one behind the other in axial direction, so that no transition piece is required. The polarization switch of the larger inner diameter has, in the case of this known antenna exciter, two sections of different inside diameters. The two wave guides are connected to respective ones of these sections. The section having the larger inside diameter adjoins the other polarization switch while the section having the smaller diameter passes into the feed horn. By this development and arrangement of the polarization switch having the larger inside diameter not only does its manufacture result in an increased expense but the construction of the entire antenna exciter also becomes expensive since the two polarization switches must be manufactured separately and be assembled with the maintaining of very close tolerances. In addition, to this, the connections for the four wave guides must be provided with transformation stages and stops of complicated design must be present in the inlet openings of the two wave guides connected to the polarization switch having the larger inside diameter. In this way, the manufacture of the antenna exciter as a whole becomes very expensive.